Congeners are minor compounds other than ethanol that occur naturally in alcohol beverages as a result of distilling and fermenting processes. While ethanol itself is the main source of hangover (subjective distress) and other residual effects of alcohol (cognitive and behavioral),
the role of the congeners is of interest due to the potential toxicity of many of them despite their minute quantities. Survey studies, while comparing beer to liquor to wine, have generally not addressed beverage effects that clearly differ in congener content. The few experimental studies
indicate that the highest congener beverage (bourbon) results in more severe hangover ratings than does the beverage with essentially no congeners (vodka), although ethanol effects per se had a considerably stronger effect on hangover than did congener content. Safety-sensitive performance
that was affected by alcohol intoxication the previous night (vigilance with reaction time; ataxia) was not differentially affected by bourbon versus vodka. The paucity of studies indicates more work is needed in order to have confidence in these results.